The present invention relates to a filter arrangement to filter signals transmitted between a first filter port and a second filter port, said arrangement including filter means adapted to filter predetermined frequency components of said signals, and associated clamping means adapted to limit the transmission of unwanted signals between said first filter port and said second filter port.
Such a filter arrangement is generally known in the art and is used to isolate the first filter port from the second filter port, and thereby the circuits, devices or apparatus connected to these ports. The signals may for instance be provided by a power supply or may be telecommunication signals whereby the filter arrangement is then used to separate different signals transmitted on a same telecommunication line but intended to different apparatus, as it is the case in a xe2x80x9cparty line xe2x80x9d system where two telephone sets share a same transmission line, e.g. connected to the second filter port. Also many telecommunication modulation schemes use Frequency Division Multiplexing (FDM). A filter arrangement is then needed to filter the signals that an apparatus, connected to the first filter port, needs from the signals it doesn""t need. Another important application of such a filter arrangement may be found in the Asymmetric Digital Subscriber Line (ADSL) systems, as well as in systems derived therefrom such as HDSL, VADSL, etc. As well known, these systems provide broadband services over a transmission line constituted by a classical twisted pair of copper wires, whereby a user may use these services without the need to wait for having a fiber connected to its premises. Since the telephone signals of the so-called Plain Old Telephone Service (POTS) are also transmitted on the same telecommunication line, a xe2x80x9csplitterxe2x80x9d is necessary to separate the high frequency ADSL signals, e.g. intended for an ADSL modem, from the lower frequency telephone signals, intended for a local telephone set. Such a splitter obviously includes a line filter arrangement. A high-pass filter is sitting in front of each modem, and a low-pass filter is sitting in front of each POTS interface. Additionally to this frequency filtering, the line filter arrangement needs further also to be adapted for isolating the apparatus connected a local port, e.g. the first filter port, from the transmission line connected a line port, e.g. the second filter port, and this during severe changes in voltage/ current from one of the sides of the arrangement. This may for instance be the case of spikes on the signals and that have to be eliminated.
A splitter including a filter arrangement is for instance described in the article xe2x80x9cADSL and VADSL Splitter Design and Telephony Performancexe2x80x9d by J. Cook et al, published in the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 13, NO. 9, DECEMBER 1995, pages 1634 to 1642. Therein is mentioned that splitters for ADSL, and more particularly the filters included therein, are preferably based on passive filters. The main reason therefore is that they obviate the power provision issues and ensure continued functioning in the event of power failure. Passive filters are also able to cope more easily with large signals, both voltage and current, inherent in telephony signaling and powering. Passive filters have a linear behavior as they mostly use inductors and capacitors, whilst active filters use operational amplifiers and resistors. Such a linear approach makes the design relatively easy because the Laplace-transforms and network technology can be used. Here again, there is a lot of literature available on these topics. However, this easy design is only valid if everything remains linear in all conditions. In practice however, coils that saturate are no longer linear. This means that big, and thus relatively expensive, coils need to be provided to take care that they do not saturate.
Another problem relating to ADSL applications is to avoid transient signals to be transmitted from the POTS to the ADSL side, and vice versa. Transient signals are disturbances produced, e.g., by polarity reversal of the battery or by starting of the ringing signal at the side of the telecommunication exchange, and/or by on-/ off-hook signals at the (user) side of the telephone set. To protect against the negative effects of the transient signals, large and expensive high (e.g. 7th) order filters are required as low-pass filters. Such transient and other over-current or over-voltage signals could, although not mentioned in the above known prior art, also be absorbed by clamping means such as Zener diodes adapted to eliminate voltage and/or current spikes. However, a problem with these known clamping means is that they operate in an abrupt way.
An object of the present invention is to provide a filter arrangement of the above known type but which is adapted to filter transient signals in a more efficient way while being simpler and cheaper than the one known from the prior art.
According to the invention, this object is achieved due to the fact that said clamping means include active device means adapted to limit variations of predetermined parameters of said signals.
In this way, the disturbances caused by transient or unwanted signals are progressively, and no longer abruptly, annihilated by the clamping means. Tests have proved that the worst transient signals are best attenuated by clamping means acting on the current variation or on the voltage variation of the signals rather than on the absolute value of these parameters. A combined limitation on the current/ voltage variations is also efficient.
In more detail, the present invention is characterized in that said filter means includes at least one reactive device having a first end connected to said second filter port and a second end coupled to said first filter port via a main path of said active device means which further has a control terminal coupled to said first end of said reactive device.
In this way, no power supply is required for the active device.
The present invention is further characterized in that said arrangement is a low-pass filter adapted to filter high frequency components of signals transmitted between said apparatus and said transmission line, in that said reactive device is an inductor, and in that said active device is a transistor.
It can be proved that, in case of ADSL signaling, the requirements on the filter means to filter-out the high frequency components of the signals are, in case the present arrangement is used, much less severe than those of the prior art: a 2nd(or at most a 3rd) order filter may now be used instead of the above mentioned 7th order filter for obtaining the same results. Moreover, the inductor used in the present 2nd order filter may be smaller than that (or those) of the prior art because it will saturate less owing to the limited current/ voltage of the signals.
In a preferred embodiment, the present invention is further characterized in that said arrangement is a low-pass filter adapted to filter high frequency components of signals transmitted between said first filter port, which has a first and a second local terminal to which said apparatus is connected, and said second filter port, which has a first and a second line terminal to which said transmission line, that is a double-wired line, is connected, in that said filter means comprises a first inductor and a second inductor as reactive device and that said active device means comprise a first transistor and a second transistor, in that said first line terminal is connected to the first end of said first inductor, the second end of said first inductor being coupled to said first local terminal via the main path of said first transistor, and the control terminal of said first transistor being coupled to said first end of said first inductor, and in that said second line terminal is connected to the first end of said second inductor, the second end of said second inductor being coupled to said second local terminal via the main path of said second transistor, and the control terminal of said second transistor being coupled to said first end of said second inductor.
In this way, the arrangement is adapted to limit either positive and negative spikes of the transmitted signals, and it operates in a fully bi-directional way.
Another characterized feature of the present invention is that said active device means further comprise a third transistor and a fourth transistor, that said first line terminal is connected to the first end of said first inductor via the main path of said third transistor, the control terminal of said third transistor being coupled to said second end of said first inductor, and that said second line terminal is connected to the first end of said second inductor via the main path of said fourth transistor, the control terminal of said fourth transistor being coupled to said second end of said second inductor.
The present arrangement then shows balanced characteristics with respect to the devices connected thereto.
The present invention is further also characterized in that said first and second line terminals are coupled via the series connected main paths of a pair of transistors of which the control terminals are connected to the first ends of said first and second inductors respectively.
The currents limited by the filter means and their associated clamping means are then derived through this pair of transistors so that the present filter arrangement remains transparent for the device connected thereto.
Further characteristic features of the present filter arrangement are mentioned in the appended claims.